Process of treating waste sulfate liquor



PROCESS OF TREA'HNG WASTE SULFATE LIQUUR Hilding Olof Vidar Bergstriirn, Stoclrsund, and Karl Gustaf Trobeck, Stockholm, Sweden, assignors to Alrtiebolaget BT-llletoder, Stockholm, Sweden, :1 company of Sweden No Drawing. Application February 17, 1948, Serial No. 9,027. In Sweden August 28, 1939 Section 1, Public Law 690, August 8, 1946 Patent expires August 28, 1959 3 Claims. (Cl. 23-48) The present invention relates to a process of treating waste sulfate liquor (black liquor) in the sulfate pulp process, prior to its concentrating by removal of water for regenerative purposes, in order to reduce losses of sulfur as well as to attain other advantages as set forth in the following.

It is previously known that waste liquor from the sulfate pulp process upon storage will change its composi tion, so that the sulfide content decreases. Our investigations have shown that this is due to an oxidation of the sulfidic components. If the waste liquor is thus subjected to a treatment with a sufiicient quantity of an oxidizing medium, e. g. air, it will, depending on the conditions under which the oxidizing treatment has been carried out, not or at least only to a small extent give ofi volatile sulfurous compounds, such as HzS, when subsequently subjected to the commonly employed evaporation in the course of its regeneration.

The usual manner in which waste sulfate cellulose liquor is at present evaporated causes great quantities of H28 to escape. The liquor will thus frequently lose from 2 up to as much as kilograms of sulfur calculated on the basis of one ton of cellulose pulp produced, in the form of H25, mercaptans and organic sulfides. The sulfur loss will partly occur during the evaporation of the waste liquor to form so called thick-lye and partly when this is dried to black stuff.

For known reasons losses of sulfur are, however, not desired in the process since the regenerated cooking liquor should contain as much NazS as possible. If the black liquor (the Waste liquor from the wood digesting process) is subjected to an oxidizing treatment according to the invention, by means of air, the sulfur will, however, become fixed in the liquor, i. e. oxidized whereby the above mentioned losses are reduced to a negligible quantity.

The oxidation may be effected by means of air in towers through which the liquor flows downwards in contact with the air, or it may take place in columns with perforated plates or hell bottoms, or in drums or other suitable apparatus in which the waste liquor is brought into contact with the air.

The oxidation, which is made prior to the conventional 2,726,927 Patented Dec. 13, 1955 concentrating of the black liquor, may take place at different temperatures and should be carried so far that the sulfide sulfur becomes oxidized at least for the main part, so that evolution of H28 during subsequent evaporation is substantially eliminated, but preferably not so far that the organic compounds present become oxidized.

By way of example may be stated that a suitable temperature of oxidation is generally above 60 C., preferably between about and C., at a concentration of about 18 to 25 per cent by weight of dry substance in the black liquor. A time of contact of from about 2 to 15 minutes has been found suitable provided that the distribution of the air in the liquor is made sufiiciently intimate, e. g. by the above described expedients, and a suzhcient amount of air is used. A convenient way of adjusting the solids content in the liquor prior to its oxidation is to add the requisite amount of already oxidized and concentrated liquor, which is thus re-cycled.

A certain alkalinity will increase the velocity of oxidation in the same way as a rise in temperature.

By the oxidation of the waste sulfate liquor according to the invention an increase in the sulfur content of the digesting liquor and a reduction in the consumption of lime in the causticizing process for regenerating the digesting liquor is obtained, and the condensate and gases formed in the evaporating step will be free or almost free from poisonous substances such as hydrogen sulfide and the like. Such a treated liquor will also cause considerably less corrosion than untreated liquor in pipes, storage tanks and particularly in the evaporators commonly employed in sulfate pulp plants.

We claim:

1. The process of regenerating black liquor in the sulfate pulp process, which comprises subjecting the liquor as it comes from the cooking step and before any substan tial evaporation thereof to a separate oxidizing step wherein it is intimately contacted with air at an elevated temperature below the boiling point of the liquor, said liquor being subjected to said oxidizing treatment without any substantial evaporation until substantially all of the sulfur present becomes fixed in the liquor without substantial loss of hydrogen sulfide, terminating said treatment before the occurrence of any substantial oxidization of the organic compounds present in said liquor, and then evaporating the thus oxidized liquor, whereby loss of sulfur compounds during the evaporating step are substantially eliminated.

2. The process of claim 1 wherein the oxidizing step is conducted by passing the black liquor downwardly in a tower in contact with the air.

3. The process of claim 1 wherein during the oxidizing step the black liquor is passed through perforated plates in contact with air, whereby foaming is produced.

References Cited in the file of this patent UNITED STATES PATENTS 2,064,953 Serpas Dec. 22, 1936 2,258,401 Badenhausen Oct. 7, 1941 2,303,811 Badenhausen Dec. 1, 1942 

1. THE PROCESS OF REGENERATING BLACK LIQUOR IN THE SULFATE PULP PROCESS, WHICH COMPRISES SUBJECTING THE LIQUOR AS IT COMES FROM THE COOKING STEP AND BEFORE ANY SUBSTANTIAL EVAPORATION THEREOF TO A SEPARATE OXIDIZING STEP WHEREIN IT IS INTIMATELY CONTACTED WITH AIR AT AN ELEVATED TEMPERATURE BELOW THE BOILING POINT OF THE LIQUOR, SAID LIQUOR BEING SUBJECTED TO SAID OXIDIZING TREATMENT WITHOUT ANY SUBSTANTIAL EVAPORATION UNTIL SUBSTANTIALLY ALL OF THE SULFUR PRESENT BECOMES FIXED IN THE LIQUOR WITHOUT SUBSTANTIAL LOSS OF HYDROGEN SULFIDE, TERMINATING SAID TREATMENT BEFORE THE OCCURRENCE OF ANY SUBSTANTIAL OXIDIZATION OF THE ORGANIC COMPOUNDS PRESENT IN SAID LIQUOR, AND THEN EVAPORATING THE THUS OXIDIZED LIQUOR, WHEREBY LOSS OF SULFUR COMPOUNDS DURING THE EVAPORATING STEP ARE SUBSTANTIALLY ELIMINATED. 